Client/server describes a relationship between two computer programs in which one program, a client, makes a service request from another program, a server, which fulfills the request. Although the client/server idea can be used by programs within a single computer, it is a more important idea in a network. In a network, the client/server model provides a convenient way to interconnect programs that are distributed efficiently across different locations. Computer transactions using a client/server model are common. For example, to check a bank account from a computer, a client program in the computer forwards the request to a server program at the bank. That program may in turn forward the request to its own client program that sends a request to a database server at another bank computer to retrieve an account balance. The balance is returned back to the bank data client, which in turn serves it back to the client in the personal computer, which displays the information to the user.
In an example client/server model, one server, sometimes called a daemon, is activated and awaits client requests. Typically, multiple client programs share the services of a common server program. Both client programs and server programs are often part of a larger program or application. Relative to the Internet, a Web browser is a client program that requests services (the sending of Web pages or files) from a Web server (which is generally referred to as a Hypertext Transport Protocol or HTTP server) in another computer somewhere on the Internet. Similarly, computer with TCP/IP installed allows a user to make a client request for files from the File Transfer Protocol (FTP) servers in other computers on the Internet.